Reference is hereby made to other related patent applications which are assigned to the same assignee as the present application; Application of O. Adlhart entitled "Fuel Cell With Multiple Porosity Electrolyte Matrix Assembly", Ser. No. 430,143, Filed on Sept. 30, 1982; Application of O. Adlhart and H. Feigenbaum entitled "Fuel Cell And System For Supplying Electrolyte Thereto", Ser. No. 430,144, Filed On Sept. 30, 1982; Application of J. Cohn, H. Feigenbaum and A. Kaufman entitled "Fuel Cell And System For Supplying Electrolyte Thereto With Wick Feed", Ser. No. 430,155, Filed On Sept. 30, 1982; and Application of H. Feingenbaum entitled "Fuel Cell And System For Supplying Electrolyte Thereto Utilizing Cascade Feed", Ser. No. 430,145, filed on Sept. 30, 1982.
This invention relates to fuel cells and, more particularly, to a fuel cell having a laminated porous matrix assembly disposed between the electrodes of the cell, the individual lamina having differing porosities for distributing electrolyte into the region between the electrodes as such electrolyte is needed during the generation of electricity.
Much research is being done in the area of fuel cell technology in order to provide ever increasing amounts of electric power and for operating such cells over longer periods of time without any need for shutdown to accomplish maintenance. As compared to other methods of generation of electric power from combustible fuels, a fuel cell has higher efficiency and is also characterized by a simplicity of physical structure in that such cells can be constructed without any moving parts.
While a variety of electrochemical reactions are known for the conversion of fuel into electricity with the direct burning of such fuels, one well-known form of cell utilizes the reaction between oxygen and hydrogen, the hydrogen serving as the fuel. One common form of construction for the hydrogen-oxygen cell is the laminated structure wherein the electrodes are spaced apart by a porous layer of material which holds an electrolyte. For example, the electrolyte may be a concentrated phosphoric acid. The hydrogen is guided by passageways behind the active region of the anode and the oxygen is guided by passageways behind the active region of the cathode. At the anode, the hydrogen gas dissociates into hydrogen ions plus electrons in the presence of a catalyst, typically a precious metal such as platinum or platinum with other metals. The hydrogen ions migrate through the electrolyte to the cathode in a process constituting ionic current transport while the electron travels through an external circuit to the cathode. In the presence of a catalyst at the cathode, the hydrogen ions, the electrons, and molecules of oxygen combine to produce water.
In order to provide for the physical placement of the respective reactants at the catalyst layers of the anode and cathode, layers of materials having hydrophilic and hydrophobic properties are disposed in an arrangement contiguous to the catalyst layers. They permit the electrolyte and the oxygen at the cathode and the hydrogen at the anode to contact the catalyst layer. The hydrophobic material is provided with pores of sufficiently large size to permit the gaseous hydrogen and the gaseous oxygen to freely flow through the material so as to come into contact with the catalyst.
Details in the construction of fuel cells, and in the component parts thereof, are disclosed in the U.S. Pat. Nos. 3,453,149 of Adlhart and 4,064,322 of Bushnell. These two patents show structures for guiding the gaseous reactants into the regions of the catalyst. In addition, the Bushnell patent shows space within a cell for the storage of electrolyte so as to compensate for any changes in the quantity of electrolyte available for ion transport. An assembly for combining together a plurality of fuel cells in a single power source is disclosed in U.S. Pat. No. 4,175,165 of Adlhart. This patent also shows a manifold for the simultaneous feeding of the reactant gases to the cathode and anode of the respective cells. The foregoing three patents are incorporated herein in their entirety by reference.
A problem arises during the operation of a fuel cell in that the cell has electrolyte losses. For instance, as a result of electrolyte volume changes, such as those due to temperature and composition changes, electrolyte can be driven out of the matrix and be permanently lost from use within the matrix. A fuel cell has limited capacity for the storage of additional electrolyte therein. Thus, depending on the amount of such storage capacity, there is limitation on the length of time during which the fuel cell can be operated before shutdown for maintenance. Such maintenance includes the replenishment of the amount of electrolyte in the requisite concentration.
A related problem is found in the distribution of electrolyte in the porous layer between the electrodes. The electrolyte is normally distributed fairly uniformly throughout the porous layer at the time of the construction of the cell. However, later, during operation of the cell, the distribution of the electrolyte can become less uniform. For example, there may be greater loss at the edges of the cell than at the central portion. Even though the porous layer has an initial charge of electrolyte, the rate of transport of electrolyte transversely through the layer is so slow as to preclude adequate compensation for the selective diminution of electrolyte at various sites along the electrodes and along the layer. In those areas wherein the electrolyte disappears completely, there results a space through which the oxygen and the hydrogen can mix with consequential damage to the cell.
An attempted solution of the foregoing problem by the use of large or smaller pores in such porous layer is of little help in solving this problem. Enlargement of the pore size reduces the capillary forces and, hence, the effectiveness of the layer as a barrier to the mixing of the gaseous reactants. Reduction of the pore size reduces the liquid transport rate and, hence, diminishes the probability of maintaining uniform distribution of the electrolyte.
Additional problems arise from the complexity of the structure required to lead the electrolyte in from a region of storage to the region of electrochemical activity alongside the layers of the catalyst. Such electrolyte lead-in structures are described in the foregoing Bushnell patent. In particular, it is noted that such structures tend to increase the size of the cell, to increase resistance losses associated with the flow of electric current, and to decrease the surface area available for the electrochemical reactions.